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SELECTIVE SUSPENSION OF TRANSMISSION FOR AVOIDING PRIORITY REVERSAL IN MOBILE AD HOC NETWORKS #R.Gunasekaran, Dr.V.Rhymend Uthariaraj Department of Information Technology, Anna University, Chennai, India #email@example.com ABSTRACT Ad hoc wireless networks are a very potential field offering lot of scope for research. In these networks, the Medium Access Control (MAC) protocols are responsible for coordinating the access from active nodes. These protocols assume greater significance since the wireless communication channel is inherently prone to such problems as hidden terminal, exposed terminal and fading effects .The scheme proposed here is used to perform priority scheduling in nodes resolving any contention scenario that can arise for the channel in the best possible manner. Alert transmission packets are used as a means of notification whenever a high priority node wants to transmit data. Suspend transmission packets are used to avoid priority reversal issue and a retry count is implemented to avoid starvation among the nodes. Keywords: Ad Hoc Networks, Alert Transmission, Suspend Transmission, Priority, Retry Count 1 INTRODUCTION A and C are hidden from each other. Now consider another case where B is transmitting to A. Since C is Contention for channel among the nodes is within B’s range it receives the transmission too and resolved using Contention based protocols. In a can eventually defer its own transmission which is heterogeneous network like ad-hoc several problems unnecessary as C’s transmission is in no way going like hidden terminal and exposed terminal problem to affect A receiving the packets from B. This is can arise. The popular Carrier Sense Multiple Access known as the exposed terminal problem i.e. C is MAC scheme and its variations such as CSMA  exposed to B. with Collision Detection (CSMA/CD) developed for wired networks, cannot be used for wireless networks. Priority scheduling is a means to avoid channel contention among the various nodes in the network. The scheme here proposes a new protocol for effective priority scheduling. Two new packets have been designed namely Alert Transmission and Suspend Transmission packets which form the crux of the new scheme. A retry count is implemented to avoid priority starvation. Figure 1: Hidden Terminal Problem The rest of this paper is organized as follows. Different flows in multi-hop networks have different Section 2 presents the related work. The proposed degree of contention. Here, the contention degree for Priority Scheduling scheme with a Suspend a flow is defined as the number of flows with which Transmission mechanism is explained in Section 3. it is competing for the channel. Two types of MAC Simulation results are given in section 4. schemes are prominently used. Reservation and contention based schemes. Reservation based 2 RELATED WORK schemes usually make some assumptions about high priority traffic. Flow scheduling is done locally 2.1 Classic CSMA problems while contention resolving probabilistically. Black- In fig 1 Node B is within the range of A and C burst dealt in  is a typical example where a high but nodes A and C are not visible to each other Let priority node transmits this black-burst signal as a us consider the case where A is transmitting to B. notification for its transmission. Reference  Node C, unaware of the transmission at B can generalizes this for wireless ad-hoc network. That is transmit data to B thus causing collision at B. This is each station can sense the transmission of the other referred to as the hidden-terminal problem, as nodes nodes in the network. Reference  explains a dynamic priority scheduling with a CAN MAC low priority packets that hear either BT1 or BT2 will protocol. defer their transmissions for some duration. In this way, channel access priority of a high priority node 2.2 IEEE MAC 802.11 DCF can be ensured. Certainly, if there is no high priority The 802.11 DCF function  is subjected to packet backlogged at a high priority node, a low several research modifications, which is giving a priority node will not receive any busy tone. back-off counter to each node such a way that every node can choose a random number between 0 to 3 PROPOSED SCHEME maximum contention window size. After sensing the channel to be idle for an inter-frame space the nodes In this scheme priority scheduling in wireless start counting their back-off counters to zero, and if ad-hoc networks, using alert transmission the channel is found to be busy they freeze the back- mechanism is implemented. This way contention for off counters. The value of Contention Window is channel access between nodes is resolved. This is constrained to be between CWmin and CWmax. A also seen to eliminate the hidden terminal and source station sends an RTS for which it receives exposed terminal problems occurring frequently in back CTS following which it transmits data and gets ad-hoc networks. Individual nodes are assigned an ACK packet back. In the event of CTS or ACK priority ‘Low and High’ based on the back off not received the source is led to believe that collision counter value. It is computed using the formula has occurred, so it is imperative that there is shown in Eq. (1) adequate waiting time for the source before it arrives at some decision. There are two waiting stages in Back Off = (1%cw)*priority*slot time …..(1) where cw is the size of the contention window for each node. And, priority is a user defined integer value. For each node slot-time is 20µs, and CWmin < CW < CWmax, where CWmin is the minimum CW, and it is usually set to 32 and CWmax is the maximum CW and often set to 1024. Figure 2: Distributed Coordination Function (DCF) ad hoc network, the inter frame space (IFS) stage and the back off stage. The back off counter is a random value between zero and the Contention Window. For example high priority source stations randomly choose the back off interval from [0, 2i+1 -1] and low priority source stations choose from [2i+1, 2i+2 -1], where i is the number of consecutive times a station attempts to send a packet. Two different values of CWmin and CWmax are set for different priority classes. It proposes an exponential increase by a factor of 2 in the event of collision. 2.3 Existing Scheme In order to effectively perform a priority scheduling among these nodes in the network, a priority scheduling scheme was proposed. Whenever a high priority packet is backlogged at some high priority node 0, it will send a primary busy tone signal every M slots before it acquires the channel, where M is a parameter of the proposed scheme. When another node1 of lower priority hears this primary busy tone signal (BT1), it will send a secondary busy tone signal (BT2). All nodes with Figure 3: Transmission of AT1 and AT2 packets Figure 5: Suspend Transmission Packet during and Data between the sender, receiver and neighbors. Priority Reversal After DIFS idle time, the station senses the Therefore in order to eliminate such a scenario, medium to determine whether or not it is idle. If it is whenever a high priority node receives a Alert idle, then the station decrements its back off value by transmission packet either directly or via indirect a slot time, otherwise the back off value stays the means it can compare the initial back off value in the same. When the back off value of a station reaches 0, Alert transmission packet to check if the source node the station sends an alert transmission packet AT1 to is of higher priority or lower than its own. The high its immediate neighbors, which again sends priority node will immediately send a SUSPEND secondary AT2 packet to its neighbors and so on TRANSMISSION (ST) packet for suspending the such that the hidden terminal problem is effectively transmission this will be directed at the source node. overcome. The transmission of AT1 and AT2 packets is shown in Fig 3. All the nodes in the However not all high priority nodes can transmit network are thus conveyed of the node’s intention to the ST packet. This transmission of ST is decided transmit data. A typical Alert transmission packet based on the following criteria: original priority of shown in Fig 4 will contain the following the node, priority threshold determined through information: Sender Address, Initial Back Off average packet transmission time. Only if a high counter of the original sender node, Receiver priority node satisfies these conditions it can transmit Addresses, and The time of transmission. the ST packet. The ST packet contains the following fields: Sender Address, Receiver Address, and Initial The frame format of the Alert Transmission Packet Back off counter, Time sent. (AT) is as follows: The frame format of the Suspend Transmission Packet (ST) is as follows: Figure 4: Frame Format of Alert Transmission Packet (AT1 or AT2) Figure 6: Frame Format of Suspend Transmission Duration represents the time of sending and TA Packet is the sender address while RA is the receiver address. Other lower priority nodes sensing the Duration represents the time of sending and TA transmission immediately freeze their back off is the sender address while RA is the receiver counters and defer their transmission to a later period. address. The right to send an ST packet for nodes will not remain constant it can be subjected to 3.1 Priority Reversal changes based on network characteristics. A priority reversal occurs when a low priority node has its back off at zero when nodes at a higher An example scenario is depicted in Fig 7. priority are in contention. This can lead to a situation Nodes 1, 4 are high priority nodes and nodes 3, 5 are where the lower priority node grabbing the channel of low priority. At t1, the initial back off values of before the Higher priority nodes. nodes 1, 3, 5 are 10, 17 and 18. DIFS+ AT1 SIFS DATA WAIT Node 1 t1 (bc=10) t2(bc=0) DIFS+ SIFS ST AT1 WAIT DATA Node 4 t3 t4 t5(bc=9)t6(bc=2) t7 t9(bc=0) AT1 DIFS + Node 3 SIFS WAIT t1 (bc=17) t2(bc=7) t5(bc=7) t6(bc=0) t8(bc=3) Node 5 t1 (bc=18) t2(bc=8) t5(bc=8) t6(bc=1) Figure 7: Scenario explaining the Priority Reversal issue with 5 nodes. excessive starvation of a low priority node. This can At t1, nodes 1, 3, 5 compete for the channel be fixed based on the number of nodes in the access while 4 stays away from contention. Once the network and network characteristics. The number of DIFS time expires, the back off time of node 1 times the back off counter is frozen is the retry count. counts to zero and then, it sends an alert transmission The backpri value of the low priority node comes in packet AT1 to all its neighbors. The nodes which handy whenever its RC value reaches a threshold. receive AT1 send AT2 packet to its neighbors. After Once a node’s retry count reaches this threshold the the SIFS period expires, nodes freeze their back off following occurs. The initial back off counter value counters. Nodes 3, 5 have their back off counters is replaced now with backpri value and a slot time is frozen at 7, 8 respectively and their retry counts are added to it. But before overwriting the initial back increased by 1. off value it is imperative that a copy of it is stored as backup in initial backup variable defined. Now the Node 1 after sending AT1 waits for a backpri value is used to overwrite the new back off DIFS+SIFS period and then takes control of the value which denotes the current or active back off channel for data transmission. At t4, nodes 4 (BC value of the low priority node. That is once RC =9), node 3, 5 contend for the channel access with 3 threshold is reached do the following beating 4 leading to a priority reversal. To overcome this, once the AT1 packet of 3 reaches node 4, it Initial backup=Initial back off (3) realizes that it has high priority than node 3. Hence, it disregards the AT1 packet and transmits a Initial back off=backpri+ slot time (4) SUSPEND TRANSMISSION packet ST to node 3 for suspending the transmission. The ST packet New back off= backpri (5) contains the current back off value, curr of the sender node i.e., node 4 in our case. Once node 3 The backpri value denotes the lowest current receives the ST packet from node 4, it resets it back back off value of the high priority nodes that have off value according to the formula. beaten the current node to access the channel. This means the low priority node is promoted to a high New Back off = curr + slot time (2) priority status temporarily, this is only fair because it has starved so long a period defined by RC threshold Where, curr is the current back off value of the to transmit the current packets, and it is necessary node sending ST. After sending ST, node 4 waits for that some means are done to promote its priority an SIFS period and then starts counting its back off status, to minimize the further backlogging of these timer to zero. Once a node receives ST it is packets. The low priority node will now enter necessary to do the following apart from resetting its contention as a high priority node since it has its back off value. Firstly, there is a variable backpri initial back off value reset. Now the initial back off initialized with the value of initial back off counter value will remain as backpri + slot-time only till the of the node. From the ST packet received, the current node transmits the current packets backlogged. The back off value of the high priority sender is obtained. RC value is reset to zero as shown in Eq. (6) and This value is compared with the existing value in initial back off value is set to the initial backup in Eq. backpri and the smaller value is stored in backpri. (7) after current packets are transmitted. Simultaneously, the back off counter is frozen. RC=0 (6) If current back off time of sender < backpri, then Overwrite backpri as follows Initial back off= initial backup (7) Backpri=current back off time of sender Else This means after the nodes are transmitted the node’s Do Nothing priority is reverted back to its original status, which is only agreeable as it cannot be promoted all the Similarly, when this low priority node gets into time. This scheme would thus be helpful in avoiding contention in the next idle phase and if it loses starvation of low priority nodes for the channel contention again by receiving an ST from a high access. priority node, it will compare the backpri value with the current back off value of the sender and store the 4. SIMULATION smaller value into backpri. Thus, the priority reversal issue is dealt with using Suspend Transmission The proposed scheme is implemented with the packets. help of ns2 and the results of the implementation analysis are illustrated in the following graphical 3.2 Starvation Avoidance representations. The Random Way Point model  A Retry Count (RC) is used to prevent the is used in ns2 simulation. Figure 8 indicates the comparisons in aggregate throughput between the observed that with the increase in the number of proposed Alert Transmission schemes to the existing nodes in the network, the throughput increases. IEEE MAC 802.11 scheme. The number of nodes is used as the measuring criteria. The simulation is 5. CONCLUSION carried out with 20, 40, 60, 80, 100 nodes. The results show that the proposed scheme produces A new priority scheduling scheme (Alert better average throughput when the number of nodes Transmission Scheme) is proposed for ad hoc increases. networks. With the use of AT1, AT2 the Alert Transmission Scheme ensures the channel access of high priority data packets. Priority reversal is also IEEE MAC 802.11 AT-ST SCHEME avoided by the use of Suspend Transmission ST 3000 packets. To avoid the starvation of lower priority 2500 packets and to ensure a fair scheduling, retry count is Aggregate Throughput used. 2000 1500 5 NODES 10 NODES 15 NODES 1000 120 500 100 THRO UG HP UT 0 80 0 20 40 60 80 100 60 Number of nodes 40 Figure 8: Comparisons in aggregate throughput 20 between proposed Alert Transmission scheme and 0 IEEE MAC 802.11 scheme. 0 0.2 0.4 0.6 0.8 1 1.2 ARRIVAL RATE Figure 9 shows the comparison results in the delivery ratio of high priority packets between the Figure 10: Throughput as a function of delay in the proposed scheme and the IEEE MAC 802.11 scheme. arrival of packets with different number of The results show that the high priority packets are nodes in network delivered at a much better rate in the proposed scheme. The average throughput is compared with the number of nodes in the network. 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